Drivelines for vehicles having at least two drive axles and, more particularly, to configurations of such drivelines exhibiting torsional vibrations.
The large trucks that are used to transport freight on our nation""s highwaysxe2x80x94including, for example, Class 8 trucksxe2x80x94are most commonly tractor-semitrailer combinations having a tractor configured with a steerable axle at the front and tandem driving axles at the rear. Typically, these trucks utilize a xe2x80x9cconventionalxe2x80x9d power train arrangement, depicted schematically in FIGS. 1 and 2. In this conventional power train arrangement, power produced by the engine (not shown) is transmitted through the transmission 108 to the forward drive axle 101 through a main drive shaft 107. Although a single drive shaft is shown, it is common, and contemplated by the present invention, that a compound main drive shaft structure (i.e., two or more drive shafts rotatably connected with universal joint(s)) may be used.
In this prior art driveline, the forward end 107A of the main drive shaft 107 connects to the output shaft of the transmission 108 with a first universal joint 109A. As used herein, xe2x80x9coutput shaftxe2x80x9d refers to a shaft, typically but not necessarily a pinion shaft, on a component such as a transmission or an axle, that is driven by or through the component to provide power to another downstream component, and an xe2x80x9cinput shaftxe2x80x9d refers to a shaft, again typically but not necessarily a pinion shaft, on a component such as an axle, which is externally driven to provide power to and/or through the component.
The rearward end 107B of the main drive shaft 107 connects to the input shaft 128 of the forward drive axle 101 with a second universal joint 109B (FIG. 2). A similar but generally shorter interaxle drive shaft 110 transmits power between the forward drive axle 101 and the rearward drive axle 102. The forward end 110A of the interaxle drive shaft 110 connects to the output shaft 130 of the forward drive axle 101 with a third universal joint 109C, and the rearward end 110B of the interaxle drive shaft 110 connects to the input shaft 132 of the rearward drive axle 102 with a fourth universal joint 109D.
The universal joints 109 allow the interconnected shafts to rotate about their respective axes, notwithstanding nonalignment between the shafts. Various types of universal joints are commonly used in automotive drivelines, but the most prevalent by far, particularly in heavy-duty applications, is the so-called cardan joint (also known as a Hooke joint). Cardan joints have the advantages of mechanical simplicity, good reliability, and low cost. A disadvantage of the cardan joints, however, is that uniform rotational motion at the input yoke of the joint results in non-uniform motion at the output yoke of the joint unless the joint operating angle is zero, that is, unless the shafts connected by the cardan joint rotate about a common axis. (The joint operating angle is defined herein to be the absolute value of the acute angle defined by the axes of the two shafts connected through the universal joint.)
The relationship between the input motion and the output motion across a simple cardan joint is well known. For small joint operating angles, a constant rotational velocity at the input yoke of the joint will produce nonconstant rotational motion at the output yoke having a maximum angular (or torsional) acceleration that increases approximately in proportion to the square of the shaft rotational speed and approximately in proportion to the square of the joint operating angle. In typical vehicle power trains, the drive shafts typically rotate at several thousand revolutions per minute. Therefore, even small joint operating angles can produce large angular accelerations. The angular accelerations are periodic, with a frequency of twice the shaft speed.
Large torsional accelerations can produce high dynamic torques on the universal joints and other driveline components. These dynamic forces can be very damaging to the internal components of the transmission, as well as the axle gearing and the universal joints themselves. Moreover, the dynamic forces are periodic, and can occur at resonant frequencies of the driveline, thereby amplifying the stresses and strains induced in the driveline. Consequently, designers strive to achieve driveline geometries with small joint operating angles that limit the torsional accelerations to levels that are consistent with long component life.
In drivelines having appropriately-phased, multiple cardan joints, non-uniform motion produced by one joint may be at least partially offset by one or more of the remaining joints. Referring now to FIGS. 3A and 3B, in a drive shaft 143 interconnecting an input shaft 141 with an output shaft 145, using a pair of cardan joints 109A, 109B, there are two configurations whereby the angular accelerations introduced at the input joint 109A will be ideally compensated for at the output cardan joint 109B. In the first ideal configuration, shown in FIG. 3A, the input shaft 141 is parallel to the output shafts 145 (xe2x80x9cparallel shaft geometryxe2x80x9d), so the joint operating angles (angles A and B) are equal. In the second ideal configuration, shown in FIG. 3B, the input shaft 141 is not parallel to the output shaft 145, but the shafts are configured so that the joint operating angles are again equal (xe2x80x9cintersecting shaft geometryxe2x80x9d). If the joint angles A and B are equal and the joints are phased appropriately, uniform rotary motion of the input shaft 141 will produce uniform rotary motion at the output shaft 145 with either the parallel shaft or the intersecting shaft geometries. However, the drive shaft 143 located between the universal joints 109A, 109B will still exhibit non-uniform motion and angular accelerations, and the inertia of the drive shaft 143 will generate second order (twice shaft speed) dynamic torsional loads on the joint assemblies 109A, 109B. If the two joint angles A and B are not equal, then uniform rotation of the input shaft 141 will produce non-uniform rotation of output shaft 145. The difference between the joint operating angles of the two joints (i.e., A minus B) is known as the xe2x80x9ccancellation error.xe2x80x9d
In order to avoid the angular vibrations introduced by cardan joints, so-called constant velocity (CV) joints have been developed. Several different types of CV joints have been developed, including, for example, ball-and-groove type joints such as Rzeppa, Weiss joints, helical or skewed groove joints, tracta joints, cross-groove joints, double-offset joints, tripot joints and flexing type joints. CV joints introduce little or no rotational non-uniformity. CV joints are commonly used in particular applications, notably on the axle shafts of front wheel drive automobiles. The primary disadvantages of CV joints are that they are complex and expensive compared to cardan joints, and they tend to have lower mechanical efficiency and poorer reliability than cardan joints. Consequently, CV joints are typically used only where acceptable performance cannot be achieved with cardan joints.
A near-constant-velocity joint can be achieved through the use of a xe2x80x9cdouble cardan jointxe2x80x9d or a xe2x80x9ccentered double cardan joint,xe2x80x9d as shown in FIG. 4. Conceptually, a centered double cardan joint 50 is made by combining two conventional cardan joints into a single joint by merging the two inner yokes into a single, two-sided xe2x80x9ccoupling yokexe2x80x9d 53. A centering bearing 57 is incorporated into the joint that constrains the operating angles of the two joint halves to remain nearly equal. While not a true constant velocity joint, the departure from ideal behavior is small until the operating angle becomes quite large.
In drive trains configured as depicted in FIG. 2, the transmission 108 is usually installed such that the axis of the transmission output shaft 126 is directed generally toward the input shaft 128 of the forward drive axle 101, such that the two universal joints 109 at either end of the drive shaft 107 operate at equivalent and small, but non-zero, joint operating angles. A joint operating angle of zero is generally avoided to ensure that the joint bearings (not shown) rotate as the drive shaft revolves in order to distribute lubrication within the bearing and avoid premature wear or xe2x80x9cbrinellingxe2x80x9d of the bearing elements.
Driveline engineers strive to achieve drive shaft geometries that provide small joint operating angles and minimal cancellation error so that second-order torsional accelerations are minimized throughout the driveline. On trucks with tandem drive axles, this has been difficult to accomplish because the two drive axles 101, 102, are closely spaced. Typically, the output shaft 130 on the forward drive axle 101 and the input shaft 132 on the rear drive axle 102 are vertically offset due to the presence of a power-dividing differential in the forward axle 101. Additionally, the drive axles 101, 102, are generally provided with a suspension system 111 that permits the axles to move vertically relative to each other, and relative to the vehicle chassis during operation of the vehicle. Because the drive axles 101, 102, are closely spaced, the joint operating angles on the interaxle shaft 110 are very sensitive to relative vertical motion between the drive axles 101, 102, caused by motion of the suspension 111.
FIG. 1 shows a trailing arm air suspension 111, of the type commonly used in modern commercial trucks, wherein the forward and rearward drive axles 101 and 102 are clamped rigidly to the trailing arm springs 103 (also called main support members). The forward end of the trailing arm springs 103 is connected with pivots 106 to frame brackets 104, which mount to frame structure 114 shown in phantom view. Air springs 105 act on the rear part of the trailing arm springs 103 and carry a portion of the sprung load. The remaining share of the sprung load is carried on the forward portion of the trailing arm springs 103. The trailing arm springs 103 have very high flexural stiffness compared to the air springs and, under normal vertical deflections of the suspensions 111, the axles 101, 102 articulate approximately about the trailing arm pivots 106.
A parallel shaft geometry driveline configuration is shown in FIG. 5, with the trailing arm springs 103 shown in phantom. Here, both the transmission 108 and the forward drive axle 101 are inclined at an angle that provides the desired small operating angles on the drive shaft universal joints 109A and 109B. The rearward drive axle 102 is installed in a parallel configuration with the forward drive axle 101. Due to the vertical offset between the forward drive axle output shaft 130 and the rearward drive axle input shaft 132, this configuration results in large joint operating angles on the interaxle drive shaft 110 at joints 109C and 109D, which produce high torsional accelerations in the interaxle shaft. Due to poor reliability of the interaxle shaft universal joints 109C and 109D, this configuration has been largely replaced with an intersecting shaft geometry, as shown in FIG. 6, which provides for smaller joint operating angles on the interaxle shaft joints 109C and 109D. In the intersecting shaft geometry, the rearward drive axle 102 is rotated such that the intersection of the axis of the forward drive axle output shaft 130 and the axis of the rearward drive axle input shaft 132 lies midway between the interaxle shaft joints 109C and 109D to provide approximately equal shaft joint angles.
The driveline geometry is usually established to provide optimal performance at the nominal operating position of the suspension 111 (shown in FIG. 1). However, the driveline must also accommodate the geometry changes that occur as a result of motions of the suspension 111. As the chassis moves up and down on the suspension 111, the axles 101 and 102 articulate about their respective trailing arm spring pivots 106 some distance ahead of the axle. Consequently, the pitch angle of the axles 101 and 102 relative to the chassis frame structure 114 (shown in phantom in FIG. 1) also changes. Both the vertical motion and the pitch rotation of the axles cause the driveline geometry and joint angles to change.
In a truck utilizing the parallel shaft geometry shown in FIG. 5, the joint operating angles on the interaxle shaft 110 will increase as the chassis frame 114 moves downward (with respect to the axles), compressing the suspension whereby the trailing arm springs 103 pivot about the pivots 106 (shown in phantom), and decrease as the chassis frame 114 moves upward. However, in a parallel shaft geometry, the two joint operating angles on the interaxle shaft 110 will remain approximately equal. So while the large joint operating angles create high torsional accelerations in the interaxle shaft 110, the lack of any significant cancellation error limits or mitigates the torsional accelerations from being propagated beyond the interaxle shaft 110.
In a truck utilizing the intersecting shaft geometry shown in FIG. 6, however, an entirely different result is obtained. As the suspension 111 (shown in FIG. 1) compresses, the joint operating angles at joint 109C at the forward end of the interaxle shaft 110 increases, while the joint operating angles at joint 109D at the rear of the interaxle shaft 110 decreases. Therefore, small movements of the suspension 111 rapidly generate large cancellation errors within the interaxle shaft 110. The cancellation error increases until one joint operating angle passes through zero, at which point the cancellation error is constant. The large cancellation error in the interaxle shaft joints 109C and 109D produces high amplitude second order torsional vibrations in the main drive shaft 107, which can damage internal components of the transmission 108 and other power train components.
As noted earlier, one possible solution for eliminating torsional vibrations is to use CV joints. Installing CV joints at both ends of the interaxle shaft 110 and at the forward drive axle input shaft 128 would largely eliminate second order torsional vibrations. However, the shortcomings of CV joints make this an expensive and unappealing solution. An additional limitation is that CV joints generally cannot operate at the large operating angles that occur in the interaxle drive shaft 110 during xe2x80x9ccross-articulationxe2x80x9dxe2x80x94i.e., when one of the drive axle suspensions is fully compressed and the other is fully extended, as often occurs when the vehicle traverses obstacles such as curbs at low speeds or otherwise operates in rough terrain. The extremely short length of the interaxle shaft 110 may also be insufficient for packaging two CV joints and also provide for the slip needed to accommodate the change in length under articulation. So the use of CV joints in the interaxle drive shafts where the largest cancellation errors occur is impractical. For these reasons, conventional highway tractors typically do not utilize CV-equipped interaxle shafts.
The present invention is directed to a low-vibration driveline for a vehicle having a plurality of rearwardly disposed, driven axles. In a preferred embodiment the low-vibration driveline includes a forward and a rearward drive axle interconnected with an interaxle drive shaft. The drive axles are oriented with the output shaft from the forward drive axle directed generally towards the input shaft on the rearward drive axle, such that the interaxle assembly is in a parallel shaft geometry with joint operating angles that are small. The interaxle drive shaft is connected to the drive axles with universal joints. A main drive shaft that transmits power to the drive axles is connected to the forward drive axle input shaft using a constant velocity, or near-constant velocity universal joint.
In a preferred embodiment, the joint operating angles on both ends of the interaxle drive shaft are not more than 5 degrees. In another preferred embodiment these joint operating angles are less than about 2 degrees.
In embodiments of the present invention that utilize constant velocity universal joints to connect the main drive shaft to the forward drive axle, the constant velocity joint may be of the Rzeppa, Weiss, tripot type, double-offset type, or derivative designs.
In an embodiment of the present invention that utilizes a near-constant velocity joint to connect the main drive shaft to the forward drive axle, a double-cardan type near constant velocity joint may be used.
In another aspect of the present invention, the forward end of the main drive shaft connects to a transmission with a universal joint, the universal joint having a small joint operating angle.
In an aspect of the present invention, the nonconstant velocity and non-near-constant velocity joints may be of the cardan type of universal joint.